In the electrophotographic method, an electric latent image is generally formed by various techniques using a photoconductive substance, e.g., selenium, as a photoreceptor and the latent image is developed by attaching tones thereto, utilizing techniques such as the magnetic brush developing method.
In this developing step, a two-component type developer is most commonly used in which carrier particles called "carriers" are used in combination with the toner to provide a suitable amount of positive or negative electric charge. Various types of carriers have been developed and put into practical use.
The carrier is required to have various characteristics. Particularly important among these characteristics are charging properties, impact resistance, abrasion resistance, developing properties, developer life, environmental independency in charge properties, and stability of charge properties with passage of time.
In view of the above characteristics, conventional carriers still remain inadequate; in fact, no sufficiently satisfactory carrier has yet been produced. For example, electrically conductive carriers, e.g., iron oxide powder, have the disadvantages that fine line reproducibility is poor, although solid developability is excellent, and further that it is necessary for a special charge controlling agent to be added to the toner in order to prolong the service life of the developer. Coated insulating carriers have the disadvantage that solid reproducibility is poor, although service life and reproducibility of fine lines are excellent. In order to overcome the above problems, small particle diameter carriers for magnetic brush development, containing a binder resin and fine magnetic particles dispersed therein, i.e., "microtoning carriers", have been proposed and put into practical use, as disclosed, for example, in U.S. Pat. No. 4,345,014 and Japanese Patent Application (OPI) No. 66134/79. (The term "OPI" as used herein refers to an "unexamined published Japanese patent application."
Such fine magnetic particle-containing carriers previously have been produced either by (1) the melt kneading pulverization method or (2) the solution spray cooling method. . Using the first method, a mixture of a resin and a powdered magnetic substance is melt kneaded and solidified, and then pulverized, after which the resulting powder is sieved to obtain carriers having the desired particle diameter. This method, however, has the disadvantage of low efficiency in producing carriers having the desired particle diameter. Furthermore, it is difficult to produce spherical carriers by this method. The carriers have irregular surfaces, and when they are used, the magnetic powder is freed by friction or contact.
In the second method, a powdered magnetic substance is dispersed in a solution of a resin in a solvent, and the resulting dispersion is sprayed while maintaining the temperature in a range at which the solvent evaporates (e.g., up to about 150.degree. C.), to produce the desired carriers. The surface of the carrier produced by this method is porous and brittle because solidification proceeds with evaporation of the solvent, making it impossible to apply a subsequent coating treatment.
As described above, conventional microtoning carriers have disadvantages such as adhesion of carriers to a photoreceptor due to their small diameter in size, changes in charging properties under high and low humidity conditions, caused by magnetic particles freed from their surface, and the further problem that since it is difficult to apply a surface treatment, it is impossible to dramatically prolong service life by coating.